The subject matter of the present invention is a process for bleaching hair in which the hair, which is treated beforehand with a composition containing an optical photosensitizer, is bleached under the influence of light, as well as means for bleaching hair under the influence of light.
As has already been known for a long time, human hair is bleached by sunlight. The degree to which the hair can be brightened depends on the original hair color and on the duration over which the sunlight is active.
However, since several days can pass before a visible bleaching effect is achieved by sunlight, even during intensive sun radiation, this method is not suitable for the intended bleaching.
A method for bleaching hair is known from U.S. Pat. No. 4,792,341 in which the hair is exposed to one or more radiation shocks of artificial light, wherein the light energy is sufficient for bleaching the melanin in the hair. The method known from the U.S. Pat. No. 4,792,341 is based on the thermal decomposition of melanin caused by the use of long-wave radiation at high intensity. The intensity of the radiation striking the hair must be selected so as to be very high in this method, and an intensive heating of the hair is observed, so that the hair can be damaged.
At present the bleaching of hair is usually effected by action of a bleaching agent on hair which contains an oxidizing agent, particularly hydrogen peroxide, wherein the bleaching action can be increased by the addition of salts of peracids.
The bleaching of hair which is used to change the natural hair color shade in a fashionable manner is chemically based on a change, e.g. an oxidative change, in the hair pigment melanin.
Although the melanin in the hair is present only in relatively small amounts, it is necessary to work with a large excess of hydrogen peroxide in the alkaline pH range when bleaching the hair with conventional bleaching agents containing hydrogen peroxide in order to ensure a sufficient bleaching effect.
In this currently conventional method for bleaching hair, hair damage cannot be avoided due to the excess of hydrogen peroxide used and the alkaline pH value.
The problem exists of providing a possibility for achieving a gentler bleaching of hair with light. A new method should be based on photochemical and not on photothermal reactions in order to prevent a heating of the hair which can damage the hair, as happens in the method known from U.S. Pat. No. 4,792,341. In addition, the new method should, be capable of bleaching the hair without an excess of oxidizing agents within a suitable period of time by means of the photochemical production of a concentration of oxidizing agent adapted to the melanin concentration in the hair.